300 
Proceedings of the Royal Society 
the second law of thermodynamics, or general equation of the 
mechanical action of heat, from the hypothesis of molecular vor- 
tices, by means of known dynamical principles. The quantity 
whose variation being multiplied by the absolute temperature gives 
the latent heat, corresponding to a given change of dimensions 
at that temperature, is expressed in Rankine’s earlier papers by 
symbols, but is not designated by a special name. 
In a paper read in January 1853 (Edin. Trans, xxi) he proposes 
the name Heat Potential ; and in a paper read to the Royal Society 
of London, January 1854, he gives to the same quantity, with a 
certain additional term, depending on changes of temperature, 
the name of “ Thermodynamic Function,” — a name which has since 
been adopted by various other authors. 
In Rankine’s paper of 1849, the chief applications of the general 
equation of thermodyamics are as follow : — The values of apparent 
as distinguished from real specific heat, for gases and vapours under 
various circumstances. The demonstration that the apparent 
specific heat of a vapour kept constantly at the pressure of satura- 
tion, while its volume varies, is negative for most fluids at ordinary 
temperature — in other words, that steam, for example, tends to 
become partially liquified when it works expansively, contrary to 
what had been previously believed. This fact was first verified 
experimentally by M. Hirn of Colmar. And the demonstration that 
the total heat of evaporation of a perfect gas increases with tem- 
perature at a rate equal to the completed specific heat of the gas 
at constant pressure. 
In the paper read December 1850, he deduced from Joule’s 
Equivalent the value 0*24 for the specific heat of air, and con- 
cluded that the previously received value 0*2669 must be erroneous. 
This was exactly verified by Regnault’s experiments, but not till 
more than three years afterwards. 
In a paper read April 1851 (Edin. Trans, vol. xx. 205) he 
deduced from the general equation of thermodynamics, as given 
in his paper of 1849, the following law of the efficiency of a 
perfect heat engine,-— that the whole heat expended is to the heat 
which disappears in \ doing mechanical work, as the absolute tem- 
perature at which heat is received to the difference between the tem- 
peratures at which it is received and rejected. 
